The present invention is particularly suitable for use in web feed tractors which feed webs at high speed and at high acceleration forces, so as to present successive lines or portions of lines for printing of characters or graphic displays by computer printers. Features of this invention may also be useful whenever feeding is accomplished by entry of one element into another so as to provide driving engagement therebetween.
Generally such tractors includes an endless belt entrained in a loop over sprocket wheels in which at least one of the sprocket wheels is driven by a drive shaft connected to equipment such as a printer on which the tractor is mounted. The belt typically has pins of round cross-section (at least at their bases), projecting from the belt to engage the perforation provided in the web to drive the web material along a linear path. The belt is normally trained over supports such as sprockets to curve away from the linear path and then return along the loop back to the linear path. Such tractors are normally employed in pairs at opposite edges of the web to drive the web along the side edges. Almost invariably the tractors are mounted parallel to each other and are drive by a common drive shaft that extends therebetween. One of the more serious problems associated with such prior art tractor device is the inaccuracy of positioning of the perforations in the web material relative to the centers of the tractor pins. This inaccuracy is usually the result of several factors: (1) inaccuracies of lateral positioning of the pins on the tractor belts; (2) inaccuracies in the alignment of the perforations in the web; (3) tolerance error in the alignment of the two tractors parallel with each other in the direction of the feed; and (4) longitudinal phase error between the pins of the two tractors to cause the lateral distance between the side perforations to vary along the length of the web material. These inaccuracies have an adverse affect on the feeding of the web material in both the transverse direction and the linear direction of the web feed.
The perforations in the paper web generally have a circular shape with a multitude of notches or serrated edge to provide a feathering engagement with the circumference of the pins. However there is generally no provision for compensation for unavoidable inaccuracies that have been previously described. The pair of tractor pins are constantly fighting for control of the paper that comes into engagement with the perforations. This leads to a jerking of the paper from one side to another which causes loss of accuracy of printing of the characters on the paper and ultimately deterioration of the print quality. It also contributes to increased level of noise produced by such devices in addition to excess wear.
Examples of typical types of tractor drives and belts are illustrated in Leo J. Hubbard; U.S. Pat. No. 3,825,162 granted July 23, 1974; Alan F. Seitz; U.S. Pat. No. 4,130,230 granted December 19, 1978 and John D. Hubbard et al U.S. Pat. No. 4,611,737 granted September 16, 1986.
The latter U.S. Pat. No. 4,611,737 provides for a mechanism for attempting to locate the perforations of the web on the belt pins as rapidly as possible so as to minimize (1) stretching of the front or rear of the perforations, (2) deformation of the perforations and (3) tearing of the perforations. To accomplish this, a shoulder on the outside of one of the lids is provided to cause the paper to ride further down on the pin than normal. Additionally the shoulder is provided with a much closer tolerance to the pin. In FIGS. 6, 7, and 7A of the patent, the pins on one belt have a flat vertical surface formed on a side surface to enable the shoulder of the lid to encroach more closely to the base of the pin. The flat side surface forms an arcuate segment of an arcuate distance of between 60.degree. to 90.degree. as illustrated by angle .beta. in FIG. 7. Such an arrangement increases wear between the lid and the pins and further increases the noise caused by temporary contact between the lid and the pins.
An additional effort has been made to increase the number of pins that are continuously in engagement with the web from 4-5 to 6-8 or more pins. Although this has provided some improvement due to a better averaging of the errors, it is not a complete solution and it is more costly because of the relatively large increase in the size and cost of the tractors.
Additionally in the past some accommodation has been made for misalignment by the use of paper in which the holes or perforations along one edge are circular in cross-section while the holes or perforations along the other edge are oval in shape. An example is illustrated in the Phillips U.S. Pat. No. 3,113,823 granted Decembrer 10, 1963. However such provision is generally limited to rather expensive papers that are used for charts. Generally cutting such perforations in the paper requires additional costs in perforating the paper with two different sets of apertures and further requires additional increase in inventory stocking of papers. It would cost the computer paper industry vast sums to convert to accommodate problems generally caused by the feed mechanism.
It is an object of this invention to overcome may of the shortcomings of the prior art by providing a novel tractor construction and a novel belt for driving the web material, including a pair of tractors which provide means for compensating for unavoidable inaccuracies that are difficult to overcome without having to utilize special paper.